1. Field of the Invention
The present invention relates generally to L band communications satellite system antennas such as that used in a Global Positioning System (GPS), INMARSAT, MSAT, PROSAT, NAVSTAR, etc.; and more particularly to a microstrip-fed cylindrical slot antenna for use in these systems.
2. Description of the Prior Art
The evolution of satellite communication networks has proceeded from the design and development stage to actual working systems within the last decade. The Global Positioning System (GPS) is one of the major accomplishments realized in systems utilizing satellite communication.
One area in which these GPS systems are utilized is in aircraft avionics. The commercial GPS user equipment for aircraft networks requires an antenna that can provide a right-hand circular polarization and a uniform pattern coverage over approximately the entire upper hemisphere. The uniform amplitude response over a wide coverage region allows the receiver to maintain signal lock to satellites with a useful signal-to-noise ratio. Because a high-speed aircraft constantly changes its look angle to satellites, the wide beamwidth coverage allows the receiver to track as many of the visible satellites as possible and maintain the system's proper Geometric Dilution of Precision (GDOP). Also, a mechanical configuration that has no appreciable drag and requires no elaborate structural modification to the aircraft is another leading concern of airborne terminal in a satellite-to-air communication link. Slot antennas are useful in applications where low-profile or flush installations are required on a high-dynamic aircraft.
The slotted cylinder antenna was first introduced by Andrew Alford in an article entitled "Long Slot Antennas," Proc. Natl. Electronics Conf., p. 143, 1946. The physical structure of the slotted cylinder antenna proposed by Alford consists of a slotted sheet metal bent into a cylinder. He described this type of vertical slotted cylinder as a resonant transmission line with a sufficient number of shunt loops. FIG. 1 shows the physical configuration of the conventional slotted cylinder antenna. As shown in FIG. 1, the antenna is formed by bending a slotted sheet metal into a cylinder 10. It should be appreciated that most of the current flows in the horizontal loops 12 around the cylinder due to the sufficiently low impedance of a circumference path around cylinder 10. A coaxial feed 14 is provided for delivering current to a radiating slot 16 as illustrated in FIG. 1. The antenna radiates a horizontally polarized field with a nearly circular pattern in the horizontal plane. This type of vertical slot antenna is suitable for broadcasting a horizontally polarized wave with an omnidirectional or circular pattern in the horizontal plane.
Cylindrical antennas have been disclosed in the prior art. For example, U.S. Pat. No. 5,353,040, by Yamada et al., discloses a four wire cylindrical antenna and a four wire stepped cylindrical antenna for use in an aircraft. Yamada clearly states that the four wire cylindrical antenna is not sufficiently broad enough for simultaneous transmission and reception through different frequency bands. This problem was overcome by Yamada by providing a step between two cylindrical antennas having different circumferences and being coaxially located. It should be appreciated that this antenna structure does not disclose a slot disposed through the cylinder.
U.S. Pat. No. 5,255,005, by Terret et al., discloses a four wire cylindrical quadrifilar helix antenna formed by two bifilar helices. As may be seen in FIG. 1, of the patent each of these helices have different diameters. Each wire, which forms a respective helix, is between .lambda./2 and .lambda. in length. It should be appreciated that this antenna structure does not disclose a slot disposed through the cylinder.
U.S. Pat. No. 5,200,757, by Jairam, discloses a cylindrical antenna having a number of parallel sided slots which extend at an angle of 45.degree. to the horn axis. These slots do not extend along the entire length of the cylinder.
U.S. Pat. No. 5,427,032, by Hiltz et al., discloses the use of a cylindrical antenna for receiving radio signals from a remote source.
U.S. Pat. No. 4,675,691, by Moore, discloses a cylindrical antenna having at least one slot disposed along the length of the cylinder as illustrated in FIG. 4.
U.S. Pat. No. 4,451,830, by Lucas et al. discloses an antenna comprising a cylindrical radiator which is formed with four orhtogonally disposed longitudinally extending slots. Each slot is backed by a separate cavity which extends into the cylinder.
U.S. Pat. No. 4,012,744, by Greiser, discloses a circularly polarized broad beam antenna system comprising a cylindrical antenna having a bifilar helix. The antenna has a planar portion which is coupled to the bifilar helix.
The radiation properties of the microstrip-fed slot antennas were first reported by Yashimura in an article entitled "A Microstrip Slot Antenna," IEEE Trans. Microwave Theory Tech., vol. MTT-20, pp. 760-762, Nov. 1972. He measured the input impedances and the radiation patterns for various geometries of microstrip-fed slot antennas. The physical structures of these slot antennas are fabricated by simple and conventional photoetching techniques and considered to be suitable for Monolithic Integrated Circuits (MIC) and Microwave Monolithic Integrated Circuit (MMIC) transceivers. They also have the advantages of being able to produce bidirectional and unidirectional radiation patterns and requiring very simple feeding and matching techniques. FIG. 2 shows the physical structure of this prior art microstrip-fed slot antenna. As shown in FIG. 2, the longer sides L of the radiating slot 18 are perpendicular to a microstrip feed line 20. The microstrip feed line 20 crosses radiating slot 18 and is short-circuited through a dielectric substrate 22. A microstrip ground 24 is disposed on dielectric substrate 22. The slot radiator can be excited either from its center or at a distance from its center. The center-fed slot antenna requires a matching circuit to match the input impedance of radiating slot 18 to the 50 .OMEGA. microstrip feed line 20. The microstrip-fed slot antenna may be modeled by a loaded transmission line.
FIG. 3 shows the equivalent circuit of the microstrip-fed slot antenna in FIG. 2. Radiating slot 18 is modeled by two short-circuited slot lines 26,28 which are loaded with a radiation resistance R.sub.s, representing radiated power from radiating slot 18. A magnetic coupling between microstrip feed line 20 and radiating slot 18 is modeled by a transformer 30. It should be appreciated that the values of turn ratio n and mutual coupling coefficient M are crucial in the determination of the input impedance. Transformer 30 is the electrical equilivent of dielectric substrate 22.
Microstrip antennas have been disclosed in the prior art. For example, U.S. Pat. No. 5,216,430, by Rahm et al., discloses a low impedance printed circuit radiating element and U.S. Pat. No. 4,612,543, by De Vries, discloses a cylindrical microstrip fed antenna mounted on a cylinder.